The invention proceeds from a high-pressure discharge lamp in accordance with the preamble of claim 1. It concerns, in particular, metal halide lamps with outer bulbs which have a base at one end. The invention is also, however, suitable for application in the case of sodium or mercury high-pressure discharge lamps.
EP-A 517 304 has already disclosed a high-pressure discharge lamp in which a discharge vessel closed at two ends is held in an outer bulb by means of a frame. In this case, both the supply lead near the base and the electric supply conductor remote from the base are sheathed by an insulating covering made from quartz glass, in order to avoid flashovers during hot restarting. Because it is assumed that a flashover chiefly takes place between these two conductors, their effective spacing is lengthened by virtue of the fact that the supply lead near the base is sheathed from its exit from the shaft of the discharge vessel virtually up to the crossing point with the associated electric supply conductor near the base. The flashover length is thereby increased.
However, it emerges in practice that the problem to be solved is more complex, since a flashover is possible between all the electrically conducting components in the outer bulb. An attempt is therefore made in general to prevent flashovers by a large spacing between oppositely polarized current-conducting components, by the avoidance of small radii of curvature and sharp bends in the conducting components, and by the use of glass tubes open at both ends, over the outer supply leads. Finally, this requires therefore relatively large volumes in the outer bulb in order to keep the spacings sufficiently large.
It is the object of the present invention to provide a high-pressure discharge lamp in accordance with the preamble of claim 1, whose capacity for hot starting is improved. This holds, in particular, for lamps of high power above 1 kW.
This object is achieved by means of the characterizing features of claim 1. Particularly advantageous refinements are to be found in the dependent claims.
In detail, the aim above all is to improve the hot-starting capacity of metal halide lamps, with a base at one end, having outer bulbs and a high electric power of preferably 4 kW to 12 kW. The lamps comprise an outer bulb having a base which is at one end and in which there is arranged a discharge vessel closed at two ends and lying on the lamp axis. It is closed by means of two sealing means, in particular seals or pinches. Immediately after the lamp is switched off, gaseous mercury and gaseous metal halides are present in the discharge vessel. These filling components bind free charge carriers. The high-voltage pulse supplied by the starting device leads in this state without special countermeasures to a discharge in the outer bulb instead of to a discharge between the electrodes in the discharge vessel. This occurs between differently polarized conductors, in particular between the two outer supply conductors, between an outer supply conductor and a lower fail system, sealed approximately at the end of the outer bulb, in a pinch, or between the supply conductor remote from the base and the opposite supply lead near the base. The fact is particularly pronounced 3 to 6 minutes after the lamp has been switched off.
A further mechanism has surprisingly proved to be disturbing, specifically a creeping discharge which forms between the free end of the supply conductor near the base and the insulating covering on the supply conductor remote from the base. The creeping discharge migrates from the covering as far as the supply conductor remote from the base itself, such that a flashover can thereby occur.
According to the invention, the free end of the supply conductor near the base is covered with respect to the covering by a cap. The cap is a hollow cylindrical one. It can be adapted in shape to the sealing part, that is to say be tubular in the case of a seal, and rectangular in cross section in the case of a pinch. The cap is open at least at the first end, which faces the discharge vessel. It is preferably closed at the second end, facing the base, particularly whenever the outer bulb is closed by means of a Moly-Cup seal, or else by means of a pinch with foils. Flashovers toward these metal parts are thereby avoided.
The free end of the supply conductor near the base is advantageously angled away transverse to the lamp axis, because then it is possible to ensure a good connection by welding or soldering to the supply conductor near the base. The cap is advantageously then slotted on one side and formed as a tube, in particular as a glass tube made from quartz glass.
The outer bulb is preferably evacuated or filled with nitrogen. A typical value for the nitrogen filling pressure (cold) is 400 to 1400 mbar, in particular 800 mbar to 950 mbar.
Overall, flashovers of any type are reliably avoided between electrically conducting, differently polarized components, and the desired flashover in the discharge vessel is thereby ensured. By comparison with the prior art, the cap permits a more compact design and a greater freedom in the construction of the lamp.
Lamps of compact design are possible because of the cap. It is already advantageously sufficient for the breakdown strength when the length of the part projecting from the discharge vessel, of the supply lead near the base to be at most six times (preferably at most three times) the diameter of the supply conductor near the base. This length or this ratio can therefore be kept extremely short/small by comparison with the prior art.
It is possible to dispense with a pinch with a foil seal as a closure technique for the outer bulb. Instead of this, use can also be made of a so-called Moly-Cup seal (see below). In this case, a cap closed at one end is preferred for the purpose of avoiding flashovers toward the molybdenum cups of the Moly-Cup seal.
The cap is preferably fastened on parts of the lamp. This is performed either by xe2x80x9carrestingxe2x80x9d it loosely between a plurality of parts, or by fastening it rigidly on one part; the covering (for example a quartz glass tube) on the long supply lead, in particular, comes into consideration for this. The cap can be fused thereon by means of an adhesive glass part (for example in the form of adhesive strips), but can also be cemented or bonded thereon.